This invention relates to a toner for use as a developing agent in an electrophotographic copier and the like and more particularly to a method of producing such a toner.
Electrophotographic copiers and the like generally use a two-component developing agent including a toner and a carrier. The toner is produced by mixing a resin material, a coloring agent such as an organic pigment and a charge controlling agent and stirring (or kneading) them together. Many kinds of organic pigments in many forms can be utilized and toners of many different colors can be produced by selecting a proper organic pigment.
It is important that the toner have uniform charging characteristics because they significantly affect the manner in which the toner particles become attached to a photosensitive body and hence the quality of the image which is eventually formed. The charging characteristics of a toner vary, however, depending significantly upon the composition of the pigment on the surface. In Japanese patent Application 61-172440 entitled (in translation) "Toner For Use In Electrophotography", the present applicants disclosed a toner which contains a pigment only in the interior section and of which the exterior layer contains a charge controlling agent dispersed therethrough such that the use of a different pigment would not affect the charging characteristics of the toner and hence that the problem mentioned above is eliminated.
With the toner described above, the charging characteristics are successfully stabilized but since its main component is a resin material and since resins have a high electrical resistance, the toner tends to become charged excessively in spite of the charge controlling agent which is also mixed together. A proper level of charging cannot be attained with such a toner. Because of this high electrical resistance, furthermore, a toner of this type requires an excessively long time for charging inside a developing tank.